The aim of the proposal is to predict both primary (before any biologic changes such as bone apposition and bone resorption take place) and secondary (when biologic changes are present) three-dimensional displacements of teeth for any force system applied to human incisors and to quantify them in terms of centers of rotation. Tooth displacements will be determined in-vivo using modern non-invasive techniques of hologram interferometry. Two holographic techniques will be used: pulsed and pulsed-sandwich, allowing measurement of varying magnitudes of tooth displacement under conditions with varying rigid-body motion of the head. Loading conditions include varying moment-to-force ratios, and force magnitudes. During loading and unloading tooth displacement velocities, sequential and cumulative tooth displacements, and sequential and cumulative centers of rotation will be determined. Patterns of primary tooth displacements will be compared to initial secondary displacements. Following secondary tooth movement, forces will be removed and relapse will be studied. As in the studies of primary and secondary displacements during loading, rates of change of displacement and centers of rotation will be determined.